In electrophotography and other forms of electrostatography a charge pattern latent image disposed on an insulating surface can be developed by contact with a dry powdered toner composition composed of finely divided charged particles of a polymer which contains a colorant. The latter can be carbon black or an organic dye or pigment which provides a desired hue for color development of the charge pattern.
For toners used to form multicolor images which are thermally fused to paper or other substrates, the colorant compound requires an unusual combination of properties. One requirement is that the colorant must be thermally stable to withstand the temperature at which it is blended with the thermoplastic toner polymer and the temperatures at which the toner is thermally fused during the image fixing step of an electrostatographic process.
Another requirement is that the colorant should not sublime excessively at the toner fixing temperature, for otherwise the optical density of the developed image would be reduced. Of course, a basic requirement is also that the colorant should have adequate tinctorial strength to provide an image of adequate density.
Still further, toners that are used in multicolor development or for making color transparencies for optical projection must form transparent images. In order for the image to be transparent the colorant must be soluble in the binder resin of the toner. Pigment particles which are insoluble in the thermoplastic polymer cause light scattering and are unsatisfactory because transparency is diminished. Soluble colorants and dyes alleviate scattering problems. The colorant must be a dye compound which is soluble in the polymer in order to provide a satisfactorily transparent image when the toner is fixed to a substrate.
In addition, for toners that are to be used for development of images on both sides of a substrate, e.g., in two-sided copying on paper, the colorant must not bleed or migrate from the toner resin. Otherwise, the image from one side of the paper sheet would appear also on the other side. The toners of this invention contain methine dyes that are immobile in the polyester binder matrix.
Of the many colorants which are useful as pigments or dyes for various purposes, as in textile dyes, paints, etc., few possess the combination of properties required for colorants to be used in transparent electrostatographic thermoplastic toners. The present invention provides toners containing a class of yellow dyes which unexpectedly have the required combination of properties, some of which dyes are novel compounds per se.